1. Field of the Invention
The present invention relates to a method of switching control for suppressing required torque and realizing high optical axis stability in a space stabilizer in an infrared imaging device and the like which is mounted on an airplane or a ship.
2. Description of the Related Art
The space stabilizer includes a so-called gimbal. The gimbal is an apparatus (mechanism) for keeping an object to be controlled such as a compass or a camera to be horizontal.
FIG. 1 is a figure for explaining a general control mode of a gimbal. The following explanation is for a control example in which the gimbal is mounted in a ship.
There are three gimbal control modes as shown in FIG. 1. The three gimbal control modes are an angle control mode with respect to ship M1, an angle control mode with respect to space M2 and an angular velocity control mode with respect to space M3. Each mode has following functions.
The angle control mode with respect to ship M1 has a function of performing positioning control for the gimbal with respect to the ship. For example, the gimbal is oriented to a predetermined housing position and is fixed by braking the gimbal.
The angle control mode with respect to space M2 has a function of correcting shaking such that the optical axis is oriented to a fixed direction in the space when disturbance is applied. According to this mode, rotation and movement of an image is suppressed, and the center of the image is always directed to the same point at infinity.
The angular velocity control mode with respect to space M3 has a function for directing the optical axis to any direction.
The operation of the gimbal from power-up to power-down is performed by switching the three control modes M1-M3 by applying control commands from the outside.
FIG. 2 shows a gimbal control flowchart.
When power is turned on in step 1, the mode becomes the angle control mode with respect to ship M1. After releasing the brake of the gimbal in step 2, the mode is changed to the angle control mode with respect to space M2 by a switching process in step 3, so that shaking correction is performed. In the angle control mode with respect to space M2, a control command from outside is received and reflected in step 4. Then, the control mode is changed to a control mode corresponding to the command by a switching process corresponding to the received command (steps 5, 6; steps 8, 9) (steps 7, 10). When a command for power-down is received in step 11, the control mode is changed to the angle control mode with respect to ship M1 in step 12, and after positioning the gimbal at an stop angle of the gimbal with respect to the ship in step 13, the brake is applied (brake ON), and, then, the power is turned of in step 14.
In the following, a configuration of a control block for suppressing control error amount and for giving higher performance to the gimbal will be described.
Generally, the control block has three-fold control loops including an angular acceleration loop, an angular velocity loop and an angle loop, in which high accuracy for positioning the optical axis can be obtained by performing response in a high frequency region.
In the following, functions of each loop will be described.
The function of the angular acceleration loop is used for quickly responding always changing required torques and for suppressing disturbance, in which the required torques include a mechanical static/dynamical friction torque which changes due to ambient temperature, a wind pressure torque against a wind receiving surface of a ship when the ship runs in wind and rain, a disturbance torque such as an unbalance torque due to vibration/impact occurred by collision between wave and the ship, an inertial torque necessary for keeping the optical axis to be stable when the ship is shaking, and the like.
The function of the angular velocity loop is used for improving tracking responsivity to the angular velocity, that is, for improving tracking response speed to the angular velocity, wherein the angular velocity indicate the angular velocity with respect to space and the angular velocity with respect to the ship in this specification.
The function of the angle loop is used for improving tracking response characteristics with respect to the angle, that is, for improving positioning ability, wherein the angle indicates an angle with respect to space and an angle with respect to ship in this specification.
A block diagram of a control system of the angle control mode with respect to ship M1 is shown in FIG. 3.
The configuration of the control block has three-fold control loops including, from inside, an angular acceleration loop 10, an angular velocity loop with respect to ship 11 in which the angular velocity with respect to ship is a feedback signal, and an angle loop with respect to ship 12 in which the angle with respect to ship is a feedback signal.
The angular acceleration loop 10 includes a subtracter 13, an object to be controlled 14 including a servo amplifier, a motor (a driving device) and a load, an sensor 15 of angular velocity with respect to ship, a multiplier 16 calculating acceleration from the angular velocity with respect to ship, and a torque observer 17. The angular velocity loop 11 includes a subtracter 18 in addition to the angular acceleration loop 10. The angle loop 12 includes a part 20 of angle instruction with respect to ship, a subtracter 20, an angle compensator 22, a multiplier 23 calculating an angle from the angular velocity and a sensor 24 of angle with respect to ship.
The subtracter 21 calculates an angle error value between the instruction 20 of the angle with respect to ship and an actual angle with respect to ship detected by the sensor 24 of angle with respect to ship, and the angle error value is compensated by the angle compensator 22. The subtracter 18 calculates an angular velocity error value between an angular velocity instruction value output by the angle compensator 22 and an actual angular velocity with respect to ship detected by the sensor 15 of angular velocity with respect to ship, and the angular velocity error value is compensated by the angular velocity compensator 19. By calculating a torque feedback signal output from the torque observer 17 from a torque instruction value output from the angular velocity compensator 19 by using the subtracter 13. Then, the result value is applied to the servo amplifier in the object to be controlled 14 as a motor driving current instruction voltage, so that the motor is driven.
FIG. 4 shows a block diagram of a control system of the angle control mode with respect to space M2.
The control block has three-fold loops 12A including, from the inside loop, an angular acceleration loop 10, an angular velocity loop 11A with respect to space in which an angular velocity with respect to space is a feedback signal, an angle loop 12A with respect to space in which an angle with respect to space is a feedback signal. The angular acceleration loop 10 in FIG. 4 has the same configuration as the angular acceleration loop 10 shown in FIG. 3. The angular velocity loop 11A with respect to space is different from the angular velocity loop 11 with respect to space shown in FIG. 3 in that an angular velocity detected by a sensor 26 of angular velocity with respect to space is applied as feedback. In the angle loop 12A, a subtracter 28 calculates a difference between an angle of the gimbal with respect to ship and a ship shaking angle 27 (a ship gyro signal), and the difference is subtracted from a target angle instruction 25 with respect to space. The ship shaking angle 27 (a ship gyro signal) is a signal which is output by a ship gyro. The ship gyro is placed at a center bottom of the ship, and the ship gyro has an inertia body of a gimbal structure having three axes rotating at high velocity. The ship gyro detects and outputs angles of inclination with respect to the gimbal three axes (role axis, pitch axis, yawing axis) by controlling so as to keep the inertia body stable with respect to space. Therefore, the ship gyro outputs angles with respect to the three axes (that is, angles of shaking of the ship).
A control system of the angular velocity control mode with respect to space M3 is shown in a block diagram in FIG. 5.
The control block has two-fold control loops including, from inside loop, an angular acceleration loop 10 and an angular velocity loop 11B with respect to space in which the angular velocity with respect to space is a feedback signal. The angular acceleration loop 10 is the same as those shown in FIGS. 3 and 4. In the angular velocity loop 11B with respect to space, a subtracter 18 subtracts the angular velocity with respect to space from an angular velocity instruction 29 with respect to space, and the result is output to the angular velocity compensator 19.
In the control modes of the three systems, only the angular acceleration loop 10 is common. Since the feedback signals and control methods used in the angle loop and the angular velocity loop are different, excessively high torque is need to be applied to the motor if the control blocks are simply switched. Thus, oscillation and divergence occur due to the excessive output torque. Therefore, it is necessary to provide a switching means for suppressing torque between the three control modes.
Generally, since the gimbal mechanism has a drive range limit in an angle of elevation with respect to ship, it is necessary to provide an operating range limit (which will be called “mecha-limit” hereinafter) in the control system such that collision can be avoided, and it is necessary to recover operation when control amount becomes within operating range.
For example, in the angular velocity control mode with respect to space M3, when continuing to provide an instruction to move the optical axis to the mecha-limit angle direction, heavy collision occurs at the mecha-limit position so that the gimbal and the driving system are damaged if a means of avoiding the collision is not provided. In addition, it is necessary to provide a means of recovering from the mecha-limit point in order to recover the optical axis within the range of mecha-limit angle.
For example, in the angle control mode with respect to space M2, when the optical axis is spatially stabilized in the vicinity of the mecha-limit, that is, when shaking is corrected, there may be cases where the optical axis can not be stabilized since shaking can not be fully corrected within the gimbal operating range according to shaking condition. In this case, the gimbal shakes with the ship in a state that the angel of the gimbal with respect to the ship does not move at the mecha-limit, and it is necessary to recover shaking correction for stabilizing the optical axis with respect to space at the time when sum of the shaking angle and the angle of optical axis with respect to space becomes within the mecha-limit range.
FIG. 6 is a figure for explaining space stabilizing function limitation in the mecha-limit angle.
In this example, it is assumed that the mecha-limit is −60° (for the sake of simplicity, assuming that the optical axis forms a depression angle of the bow), and that shaking disturbance of ±10° is applied in a state that the angle of the optical axis with respect to space is −55°. The optical axis is spatially stabilized such that the optical axis is directed to a target when the gimbal is in the gimbal operating range. The gimbal is stopped with respect to the ship at the mecha-limit point, and shaking correction is recovered at the time when the gimbal comes into a target trackable range.
In the angle control mode with respect to ship M1, the gimbal is controlled such that the angle instruction value with respect to ship does not exceed the mecha-limit.
Following methods have been proposed as conventional switching methods between control modes of the three control systems shown in FIGS. 3-5.
A first conventional example of the switching method between the control modes is a method in which the control modes are switched by using the angle control loop. A control block of this first conventional example is shown in FIG. 7. In FIG. 7, the control block includes a switch (SW) 30, an angle instruction generation part 31, a subtracter 32, an angle compensator 33, a motor amplifier 34, a motor and load part 35, an integrator 36, a ship shaking angle 37, a switching judgment part 38, an adder 39 and an angle sensor with respect to ship 40.
In the angle control mode with respect to ship M1, the angle instruction generation part 31 outputs a target angle with respect to ship as an instruction angle in a state that the ship shaking angle 37 is not reflected by turning off the switch 30. In the angle mode with respect to space M2, the angle instruction generation part 31 outputs a target angle with respect to space as an instruction angle in a state that the ship shaking angle 37 is reflected by turning on the switch 30. For switching from the angle control mode M1 to the angle control mode M2, the switching judgment part 38 turns on the switch 30 for connecting the ship shaking angle 37 so that the gimbal is controlled for ship shaking. Normally, in order to improve tracking response ability at the start of connection, the switching judgment part 38 is used for connecting the ship shaking angle 37 when the gimbal angle error with respect to space is small.
In addition, when the angle control mode with respect to space M2 is switched to the angle control mode with respect to ship M1, the switching judgment part 38 turns off the switch 30 so as to disconnect the ship shaking angle, then, the angle of the gimbal with respect to the ship is controlled from the angle at the time of switching to the target retracting position by an angle instruction signal with respect to ship from the angle instruction generation part 31.
This method does not include the angular velocity control mode with respect to space M3. However, the optical axis can be directed to any direction by changing the instruction signal from the angle instruction generation part 31.
A second conventional example is a method of switching between the angle control and the angular velocity control, which is a servo control system disclosed in Japanese laid-open patent application No. 6-289937. A control block when the second conventional example is applied to this system is shown in FIG. 8. This control block includes an angle generation instruction part 31, a motor amplifier 34, a motor and load part 35, an integrator 36, a ship shaking angle 37, an angle sensor 40 with respect to ship, an angular velocity generation part 41, an angular velocity compensator 42, a switch (SW) 43, an angular velocity sensor 44 with respect to space, an adder 45, a subtracter 46, an angle compensator 47, a drift correction angle compensator 48, an adder 49 and a switching judgment part 50.
In the angle control mode with respect to ship M1, the switching judgment part 50 switches the switch 43 to the side of the angle control mode with respect to ship M1, and an angle instruction value with respect to ship from the angle instruction generation part 41 is output by using the angle sensor 40 with respect to ship so that the angle with respect to ship is controlled toward the target value.
In the angle control mode with respect to space M2, the switching judgment part 50 switches the switch 43 to the side of the angle control mode with respect to space M2, and an angular velocity instruction value with respect to space from the angular velocity instruction generation part 41 is output by using the angular velocity sensor 44 with respect to space so that the angular velocity with respect to apace is controlled toward the target value.
When the angle control mode M1 with respect to ship is switched to the angle control mode M2 with respect to space, the switching judgment part 50 switches the switch 43 to the angle control mode M2, and angular velocity control with respect to space is performed toward a target value which is the angular velocity instruction value with respect to space from the angular velocity instruction generation part 41 by using the angular velocity sensor 44 with respect to space.
Normally, the angular velocity sensor 44 with respect to space includes drift component. Therefore, it is necessary to form an angle loop in order to correcting the drift, in which the adder 49 adds the angle sensor 40 and the ship shaking angle 37 and a control constant of the drift correction angle compensator 48 is set such that response bandwidth becomes low frequency by which the drift can be removed.
Normally, for switching of the control modes, in order to improve tracking response ability at the time of connection start, the switching judgment part 50 connects a signal and tracks the ship shaking angle 37 after waiting for a difference between an angle instruction voltage and an angular velocity instruction voltage to become constant within an allowed range in a specified time.
In addition, in order to respond to torque shaped like step at the time of switching between the angle control mode and the angular velocity control mode, there are cases where gains of the angular velocity compensator 42 and the angle compensator 47 are decreased, or the gain of the angular velocity compensator 42 and the angle compensator 47 are changed from a state of decreased gain to an established gain.
In a third conventional example of the switching control method in the vicinity of the gimbal mecha-limit, an electrical limit switch, for example, is provided in the mecha-limit position, in which driving limitation is provided by using an electrical circuit such that, when a stopper pushes the electrical limit switch, the gimbal does not rotate in the pushing direction. There is a case where an angle signal with respect to ship is used as a judgment reference angle instead of using the electrical switch.
FIG. 9 shows a figure for explaining a limit control function according to the third conventional example.
In a driving mechanism which includes a limit plate 51 and rotates about the axis in the directions of CW (clockwise)/CCW (counterclockwise), two limit switches SW1 and SW2 are provided in fixed parts for detecting upper and lower mecha-limit angles. When the mechanical part reaches a limit point, the limit plate 51 pushes the switch SW1 or the switch SW2, and an instruction voltage output is restricted such that the limit plate does not rotate to the direction of the pushed switch for avoiding collision.
However, there are following problems in the first to third conventional examples.
The problem of the first conventional example is as follows.
The first conventional example is a cheap and simple method for correcting gimbal shaking. Since an angular velocity sensor is not used, the structure is simple. However, accuracy of positioning is bad, and response speed is low. In addition, there are problems in that, it is necessary to use a large torque motor which can output a torque for tracking response to angular velocity disturbance which is applied like steps, and the bore or the length of the motor becomes large. By using the switching judgment part, rising torque can be suppressed to some extent. However, a switch waiting time becomes necessary, and it may occur that switching start time becomes long according to a ship shaking condition. In addition, there is a problem in that tracking operation becomes unstable due to that a ship gyro signal shaped like step is applied when switching.
Problems of the second conventional example is as follows.
FIG. 10 shows a relationship between the angular velocity with respect to ship and the angle with respect to space when operation of the gimbal is spatially stable. Since phases of the angle control and the angular velocity control are different by 90°, the speed becomes maximum in a state where the gimbal angle with respect to ship and the shaking angle with respect to ship are almost the same (normally, tracking starts from a position where the angle with respect to ship is 0°) when the angle control mode with respect to ship is switched to the angular velocity control mode with respect to space. Therefore, large torque is necessary for switching in a shaking condition. Thus, switching process is difficult. Therefore, this method is suitable for the airplane and the like in which shaking is small. For the second conventional example, a large torque motor which can output torque for tracking response to angular velocity disturbance which is applied like steps is necessary. Thus, the gimbal becomes large. Comparing with the first conventional example, the space stabling ability is medium.
In addition, normally, since drift is included in the angle sensor itself, there is a problem in that the optical axis is drifted when control by the angular velocity instruction is performed. In order to avoid this problem, it is necessary to form an angle loop of low response bandwidth outside of the angular velocity loop.
By using the switching judgment part, it is possible that the rising torque can be suppressed to some extent. However, a time for waiting the start of switching by the judgment part is required, and a margin for the switching range used for switching judgment is necessary. Therefore, the step-like disturbance can not be removed so that tracking operation becomes unstable.
Problems of the third conventional example is as follows.
Although this method is a general method for restricting operation in the vicinity of mecha-limit, large step-like torque occurs due to deceleration/acceleration when stop/retracking occurs for switching at the limit point. Therefore, smooth stop/smooth retracking can not be performed, so that the gimbal may oscillate in some cases when switching is performed. Thus, it is necessary to use a large motor which can output torque for tracking the response. Therefore, the gimbal becomes large.
In the conventional methods of the first and second methods, since tracking is performed according to judgment condition of the switching processing part, high speed response ability for tracking is not realized. In addition, since the control is performed only by the angle loop and the angular velocity loop, the gimbal control error becomes large so that high performance can not be obtained.
There is a method for downsizing the motor other than the above-mentioned methods in which a speed reducer is used. However, there is a defect in that a positioning space of the speed reducer is necessary, response performance for the angle, the angular velocity and the angular acceleration is sacrificed.